1. Field of the Invention
The present invention relates to a radio frequency switching apparatus applied to a mobile telecommunication terminal such as a GSM (Global System for Mobile Communication) terminal, a DCS (Digital Communication Services) terminal, a PCS (Personal Communication Services) terminal or etc., and more particularly to a radio frequency switching apparatus wherein a filter for a transmitter module passes a transmission frequency to an antenna in a transmit(Tx) mode and functions as a ¼ wavelength impedance transformer in a receive(Rx) mode to isolate the antenna and the transmitter module from each other in an alternating current (AC) manner, so that no separate ¼ wavelength strip line is required while performances, such as a loss characteristic of a receiver module, and an isolation characteristic of the transmitter module are satisfied, thereby making the circuitry of the switching apparatus simpler and, thus, the size thereof smaller, and a mobile telecommunication terminal using the same.
2. Description of the Related Art
Generally, mobile telecommunication terminals have a variety of functions and the sizes and designs thereof are greatly taken into account. In this regard, radio frequency (RF) associated components, such as an RF switch, etc., have to meet demands for miniaturization, modularization and multifunctionalization.
FIG. 1 illustrates an operational conception of a general RF switch which is applied to a digital mobile phone or etc. As shown in this drawing, the RF switch is connected in common to an antenna ANT, transmitter module TX and receiver module RX to switch over the connection of the transmitter module TX to the antenna ANT and the connection of the receiver module RX to the antenna ANT to each other. The transmitter module TX generally includes a low pass filter (LPF) connected to the antenna ANT, and a transmitter circuit TX′ connected to the LPF.
One example of such conventional RF switches is disclosed in Japanese Patent Laid-open Publication No. Heisei 07-79173. This conventional RF switch will hereinafter be described with reference to FIG. 2 along with an LPF disclosed in the same publication.
FIG. 2 is a circuit diagram of the above-mentioned conventional RF switch. As shown in this drawing, the conventional RF switch, denoted by the reference numeral 11, includes a first diode 4a having its anode connected to a transmitter circuit TX′ and its cathode connected to an antenna ANT, a first strip line 5a connected between the anode of the first diode 4a and a ground terminal, a second strip line 5b connected between the antenna ANT and a receiver module RX, a second diode 4b having its anode connected between the second strip line 5b and the receiver module RX and its cathode connected to the ground terminal, and a third strip line 5c connected between a connection point of the second strip line 5b and second diode 4b and the receiver module RX.
An LPF is connected between the transmitter circuit TX′ and the RF switch 11 to pass a transmitted signal from the transmitter circuit TX′ to the antenna ANT.
In the RF switch 11, the first strip line 5a and the second strip line 5b each have a length set to ¼ of the wavelength λ of a transmission frequency, and the sum of the lengths of the second strip line 5b and third strip line 5c is set to ¼ of the wavelength λ of a reception frequency.
In the conventional RF switch with the above-described construction, in a transmit(Tx) mode under the condition that the reception frequency is lower than the transmission frequency, voltage levels at control terminals 6a and 6b become higher than or equal to turn-on voltage levels, so the second diode 4b is turned on, thereby causing the connection point of the second strip line 5b and third strip line 5c to be grounded. At this time, the third strip line 5c is equivalently isolated from a transmission line so as to have no effect on the transmitter circuit TX′, resulting in impedance matching with the transmitter circuit TX′. In a receive(Rx) mode under the same condition, the second diode 4b is turned off, so the second strip line 5b and the third strip line 5c are connected in series. As a result, the second strip line 5b and the third strip line 5c are impedance-matched with the receiver module RX because the sum of the lengths thereof is set to ¼ of the wavelength λ of the reception frequency.
Note that a strip line having a length set to ¼ of the wavelength λ of the transmission frequency must be provided in the conventional RF switch in association with the receiver module for the switching between transmission and reception by the RF switch. For example, assuming that the transmission frequency is 1 GHz, the wavelength λ thereof is about 30[cm], as can be obtained from the following equation 1:
                                                                        Wavelength                ⁢                                                                  ⁢                                  (                  λ                  )                                            =                            ⁢                                                luminous   flux (                  c                  )                                /                                  frequency (                  f                  )                                                                                                        =                            ⁢                              3                *                                  10                  8                                ⁢                                                                  ⁢                                  [M]/1  GHz                                                                                                        =                            ⁢                              3                *                                  10                  10                                ⁢                                                                  ⁢                                  [cm]/                                ⁢                1                *                                  10                  9                                                                                                        =                            ⁢                                                30                  ⁢                                      /                                    ⁢                  1                  ⁢                                                                          ⁢                                      [cm]                                                  =                                  30                  ⁢                                                                          ⁢                  cm                                                                                        [                  Equation          ⁢                                          ⁢          1                ]            
Accordingly, the value of ¼ of the wavelength λ of the transmission frequency 1 GHz corresponds to a length of 7.5 (30/4=7.5) [cm]. In order to implement an inductor having that length as a strip line, it is necessary to form substrates with patterns of certain lengths in a multilayered manner and electrically connect the patterns of the multilayered substrates with one another.
However, in the case where the RF switch is made in the form of a module, the ¼ wavelength strip line substantially occupies a large volume on the RF switch module, corresponding to about ⅓ of the entire volume of the switch module. For this reason, the RF switch, which essentially includes the ¼ wavelength strip line, is increased in size and complicated in construction, leading to a limitation in miniaturizing the RF switch.
Particularly, in the case where the RF switch is made in the form of one module, including the LPF which passes the transmission frequency to the antenna, it is still more increased in size.